Bottom Line:
In agreement with previous models and experiments, icosahedral-like clusters are observed.The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging.In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable.

ABSTRACTThe structure of metallic glasses is a long-standing question because the lack of long-range order makes diffraction based techniques difficult to be applied. Here, we used scanning tunneling microscopy with large tunneling resistance of 6 GΩ at low temperature in order to minimize forces between probe and sample and reduce thermal fluctuations of metastable structures. Under these extremely gentle conditions, atomic structures of Ni40Ta60 metallic glasses are revealed with unprecedented lateral resolution. In agreement with previous models and experiments, icosahedral-like clusters are observed. The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging. In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable. The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

f3: DFT calculations of metastable structures in the Ni40Ta60 glass.(a) Stable Ni3Ta10 icosahedral structure, Ni and Ta atoms are colored in green and blue respectively. (b) Maximum and average diameters of NinTa13−n clusters as a function of their stoichiometry (factor n). All NinTa13−n preserve their initial icosahedral geometry after relaxation. (c) Stable structure of a linear Ni8Ta21 chain, the distance between Ta atoms is 0.35 nm in agreement with STM observations. (d) Example of a metastable Ni8Ta21 chain structure after 100 relaxations obtained by minimum hopping method combined with DFT. Among all relaxations, the chains reveal an important structural adaptability. e, Histogram of inter-atomic distances for about 100 low energy configurations of Ni7Ta18 and Ni8Ta21 chains.

Mentions:
To better understand the observed structural motifs, we performed the minima hopping method coupled to Density Functional Theory (DFT) calculations30 (see details in Methods) in order to find the most stable and low energy configurations of both icosahedral and chain-like structures. In agreement with the STM data, we focused on icosahedral clusters (Fig. 3a) and relaxed Ni7Ta18 and Ni8Ta21 chains structures (Fig. 3c,d). First, we considered NinTa13−n icosahedral clusters consisting of 13 atoms which have sizes similar to the STM data Fig. 2e. After relaxation, all configurations preserve the initial icosahedral structures as depicted in Fig. 3a. The graph Fig. 3b shows the variation of their diameters depending on their stoichiometry which varies between between 0.44 and 0.54 nm. These values are in good agreement with the STM observations of ≈0.6 nm, which is slightly larger most likely due to tip-sample convolution artifact. Although we cannot conclude to a particular Ni/Ta stoichiometry, the icosahedral geometry is always observed at the surface of the MG. Figure 3c depicts the stable and unrelaxed Ni8Ta21 chain having a tetragonal structure as obtained by the minima hopping method with BigDFT31. The distance between neighboring Ta atoms is 0.38 nm and confirms the inter-unit distance a measured between high contrast spots along the chains of the STM image (Fig. 2f). Figure 3d shows a characteristic chain structure obtained after several relaxations using minima hopping method (see Methods). The distance between Ta atoms is marked along the chain and shows important variations. The histograms Fig. 3e shows the Ni-Ni, Ta-Ta atomic distances for about 100 low-energy chains with relaxed and stable configurations respectively. The broad peak at ≈0.4 nm on the blue histogram coincides with the Ta-Ta inter-unit distance along the chain. Its large distribution shows the important variation of the bond lengths between atoms in the chain and reveals that the structure can be strongly deformed while remaining stable, thus favoring a close packed arrangement in the metallic glass.

f3: DFT calculations of metastable structures in the Ni40Ta60 glass.(a) Stable Ni3Ta10 icosahedral structure, Ni and Ta atoms are colored in green and blue respectively. (b) Maximum and average diameters of NinTa13−n clusters as a function of their stoichiometry (factor n). All NinTa13−n preserve their initial icosahedral geometry after relaxation. (c) Stable structure of a linear Ni8Ta21 chain, the distance between Ta atoms is 0.35 nm in agreement with STM observations. (d) Example of a metastable Ni8Ta21 chain structure after 100 relaxations obtained by minimum hopping method combined with DFT. Among all relaxations, the chains reveal an important structural adaptability. e, Histogram of inter-atomic distances for about 100 low energy configurations of Ni7Ta18 and Ni8Ta21 chains.

Mentions:
To better understand the observed structural motifs, we performed the minima hopping method coupled to Density Functional Theory (DFT) calculations30 (see details in Methods) in order to find the most stable and low energy configurations of both icosahedral and chain-like structures. In agreement with the STM data, we focused on icosahedral clusters (Fig. 3a) and relaxed Ni7Ta18 and Ni8Ta21 chains structures (Fig. 3c,d). First, we considered NinTa13−n icosahedral clusters consisting of 13 atoms which have sizes similar to the STM data Fig. 2e. After relaxation, all configurations preserve the initial icosahedral structures as depicted in Fig. 3a. The graph Fig. 3b shows the variation of their diameters depending on their stoichiometry which varies between between 0.44 and 0.54 nm. These values are in good agreement with the STM observations of ≈0.6 nm, which is slightly larger most likely due to tip-sample convolution artifact. Although we cannot conclude to a particular Ni/Ta stoichiometry, the icosahedral geometry is always observed at the surface of the MG. Figure 3c depicts the stable and unrelaxed Ni8Ta21 chain having a tetragonal structure as obtained by the minima hopping method with BigDFT31. The distance between neighboring Ta atoms is 0.38 nm and confirms the inter-unit distance a measured between high contrast spots along the chains of the STM image (Fig. 2f). Figure 3d shows a characteristic chain structure obtained after several relaxations using minima hopping method (see Methods). The distance between Ta atoms is marked along the chain and shows important variations. The histograms Fig. 3e shows the Ni-Ni, Ta-Ta atomic distances for about 100 low-energy chains with relaxed and stable configurations respectively. The broad peak at ≈0.4 nm on the blue histogram coincides with the Ta-Ta inter-unit distance along the chain. Its large distribution shows the important variation of the bond lengths between atoms in the chain and reveals that the structure can be strongly deformed while remaining stable, thus favoring a close packed arrangement in the metallic glass.

Bottom Line:
In agreement with previous models and experiments, icosahedral-like clusters are observed.The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging.In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable.

ABSTRACTThe structure of metallic glasses is a long-standing question because the lack of long-range order makes diffraction based techniques difficult to be applied. Here, we used scanning tunneling microscopy with large tunneling resistance of 6 GΩ at low temperature in order to minimize forces between probe and sample and reduce thermal fluctuations of metastable structures. Under these extremely gentle conditions, atomic structures of Ni40Ta60 metallic glasses are revealed with unprecedented lateral resolution. In agreement with previous models and experiments, icosahedral-like clusters are observed. The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging. In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable. The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.